16 - Origins of Bipedalism

Q: How is the word hominin used in this course?

A: Hominin refers to all species on the human lineage after the split from chimpanzees (e.g., Sahelanthropus → Homo sapiens).

Q: Why is understanding taxonomic terms important?

A: It allows you to interpret scientific literature, understand evolutionary relationships, and follow how fossils are categorized.

Q: What are the major primate groups we focus on?

A: Strepsirrhines, haplorhines, anthropoids, platyrrhines, catarrhines, cercopithecoids, hominoids, and hominins.

Q: What genera belong to the early hominins?

A: Sahelanthropus, Orrorin, Ardipithecus, Australopithecus, Paranthropus, and Homo.

Q: What is bipedalism?

A: Locomotion on two feet.

Q: What is the difference between habitual and obligate bipedalism?

A: Habitual: regularly uses bipedalism.
Obligate: anatomy requires bipedal gait; cannot efficiently use another mode (modern humans).

Q: What is striding bipedalism?

A: The fully committed, energy-efficient form of bipedal walking unique to humans.

Q: Do other animals show bipedalism?

A: Yes, but mostly facultative or temporary; humans are the only obligate, striding bipeds.

Q: What is the Postural Feeding Hypothesis?

A: Bipedalism evolved from upright posture while feeding in trees; supported by observations of apes.

Q: Problem with the Postural Feeding Hypothesis?

A: Other terrestrial primates remained quadrupedal.

Q: What is the Thermoregulation Hypothesis?

A: Bipedalism reduces solar exposure and increases airflow in open environments.

Q: Problem with Thermoregulation Hypothesis?

A: Many quadrupeds thrive in open, hot environments without becoming bipedal.

Q: What is the Free Hands Hypothesis?

A: Bipedalism evolved to free hands for carrying food, tools, or infants.

Q: Problem with Free Hands Hypothesis?

A: Tool use and altriciality appear after early bipedalism.

Q: What is the Savanna (Aridity) Hypothesis?

A: Expansion of savannas favored efficient locomotion on the ground.

Q: Problems with the Savanna Hypothesis?

A: Early bipeds lived in forested environments; grasslands expanded later.

Q: What is the Vigilance Hypothesis?

A: Bipedalism evolved to spot predators over tall grass.

Q: Problem with the Vigilance Hypothesis?

A: Other mammals do not evolve bipedalism for vigilance.

Q: What is the Threat Display Hypothesis?

A: Bipedalism evolved from standing upright during aggressive displays.

Q: Problem with the Threat Display Hypothesis?

A: Threat displays work better when rare, not constant behavior.

Q: What is the Energetics Hypothesis?

A: Bipedalism evolved as a more energy-efficient mode of travel.

Q: Problem with the Energetics Hypothesis?

A: Energy efficiency benefits are debated.

Q: What is the Terrestrial Food Gathering Hypothesis?

A: Early hominins stood upright to dig for underground storage organs (USOs).

Q: Problem with the Terrestrial Food Gathering Hypothesis?

A: Quadrupedal primates can also access USOs.

Q: Can bipedalism be explained by a single hypothesis?

A: No. Likely a combination over time—early selection for efficient terrestrial movement, later benefits for tool use and foraging.

Q: What skull adaptation supports bipedalism?

A: Anteriorly placed foramen magnum to balance head over spine.

Q: What spinal features support bipedalism?

A: S-shaped curve, lumbar enlargement, and vertebrae that increase in size downward.

Q: What is the bicondylar (valgus) angle?

A: The inward angling of the femur that positions knees under the body.

Q: Why is the tibial plateau expanded in bipeds?

A: To support greater weight and absorb shock.

Q: What is the significance of symmetrical femoral condyles?

A: Creates stable, efficient knee movement during walking.

Q: Key foot traits of bipeds?

A:

• Short toes

• Robust, non-opposable big toe

• Rigid midfoot

• Arches (longitudinal + transverse)

• Strong heel (calcaneus)

Q: Why are foot arches important?

A: They provide shock absorption and efficient push-off.

Q: What pelvic changes support bipedalism?

A: Short, bowl-shaped pelvis; reoriented iliac blades; improved gluteal muscle leverage.

Q: Why do bipeds have laterally oriented shoulder joints?

A: Reduced climbing adaptations; shoulders repositioned for stability, not suspension.

Q: What does a low humero-femoral index indicate?

A: Longer legs relative to arms—adaptation for walking.

Q: What does obligate bipedalism mean?

A: A locomotor form where the anatomy requires two-legged walking as the primary mode of movement.

Q: Can you list the major hypotheses for bipedalism?

A: Postural Feeding, Thermoregulation, Free Hands, Savanna/Aridity, Vigilance, Threat Display, Energetics, Terrestrial Food Gathering.

Q: What are problems with the eight hypotheses?

A: Each has limitations (e.g., contradicting environments, quadrupedal comparisons, incorrect timing relative to fossil record).

Q: What is the best explanation for bipedalism?

A: A combination: early selection for efficient terrestrial movement (Late Miocene), later enhanced by free hands, changing environments, and new foraging strategies.

Q: What are the major skeletal traits of bipedality?

A:

• Foramen magnum anterior

• S-shaped spine

• Valgus knee

• Enlarged tibial plateau

• Arched foot

• Non-opposable big toe

• Bowl-shaped pelvis

• Longer legs

• Reduced climbing adaptations

Q: How does each trait relate to function?

A: They improve balance, shock absorption, energy efficiency, stability, and forward propulsion during upright walking.